The second human that the First Doctor and Ian Chesterton encountered in the Sensorite caverns was a member of an exploration mission from the Earth Empire.
The humans made contact with the Sensorites, discovered that the Sense Sphere was rich in molybdenum and attempted to exploit its mineral wealth. Shortly afterwards, two of the crew deserted the them in the spaceship, which the Commander destroyed. The surviving three crewmembers hid in the caves outside the Sensorites' city. They began adding atropine to their water supply in the hopes of killing them all and claiming the planet for themselves. Their scheme was discovered by the Doctor and they were taken into custody by Maitland's crew. (TV: "A Desperate Venture")
